The H1N1 swine flu virus was first identified in people in April 2009 but genetic research later suggested it had in fact been circulating for at least a decade and probably longer in pigs.
This pandemic H1N1 (virus) has this mutation and is why it can replicate so well in humans, wrote Yoshihiro Kawaoka of the University of Wisconsin-Madison's School of Veterinary Medicine and the University of Tokyo, who co-authored the paper.
This gives us another marker to help predict the possibility of future flu pandemics.
Typically, a flu virus needs two amino acids -- lysine and asparagines -- on specific sites on its structure before it can jump from animals to people and multiply efficiently in human cells.
But the H1N1 remained a puzzle for scientists because they could not find the amino acids in those two locations.
In an experiment with mice, Kawaoka and colleagues discovered the lysine amino acid was residing instead in a completely different position, but it allowed the virus to be just as effective in adapting to human cells.
The World Health Organization said early in June the H1N1 pandemic was not yet over, although its most intense activity has passed in many parts of the world.
Children and people with underlying conditions that weaken the immune system such as asthma, diabetes, heart disease and pregnancy have been the hardest hit.